--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_s_mp_invmod_slow.c	Tue May 26 17:36:47 2020 +0200
@@ -0,0 +1,119 @@
+#include "tommath_private.h"
+#ifdef BN_S_MP_INVMOD_SLOW_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* hac 14.61, pp608 */
+mp_err s_mp_invmod_slow(const mp_int *a, const mp_int *b, mp_int *c)
+{
+   mp_int  x, y, u, v, A, B, C, D;
+   mp_err  err;
+
+   /* b cannot be negative */
+   if ((b->sign == MP_NEG) || MP_IS_ZERO(b)) {
+      return MP_VAL;
+   }
+
+   /* init temps */
+   if ((err = mp_init_multi(&x, &y, &u, &v,
+                            &A, &B, &C, &D, NULL)) != MP_OKAY) {
+      return err;
+   }
+
+   /* x = a, y = b */
+   if ((err = mp_mod(a, b, &x)) != MP_OKAY)                       goto LBL_ERR;
+   if ((err = mp_copy(b, &y)) != MP_OKAY)                         goto LBL_ERR;
+
+   /* 2. [modified] if x,y are both even then return an error! */
+   if (MP_IS_EVEN(&x) && MP_IS_EVEN(&y)) {
+      err = MP_VAL;
+      goto LBL_ERR;
+   }
+
+   /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
+   if ((err = mp_copy(&x, &u)) != MP_OKAY)                        goto LBL_ERR;
+   if ((err = mp_copy(&y, &v)) != MP_OKAY)                        goto LBL_ERR;
+   mp_set(&A, 1uL);
+   mp_set(&D, 1uL);
+
+top:
+   /* 4.  while u is even do */
+   while (MP_IS_EVEN(&u)) {
+      /* 4.1 u = u/2 */
+      if ((err = mp_div_2(&u, &u)) != MP_OKAY)                    goto LBL_ERR;
+
+      /* 4.2 if A or B is odd then */
+      if (MP_IS_ODD(&A) || MP_IS_ODD(&B)) {
+         /* A = (A+y)/2, B = (B-x)/2 */
+         if ((err = mp_add(&A, &y, &A)) != MP_OKAY)               goto LBL_ERR;
+         if ((err = mp_sub(&B, &x, &B)) != MP_OKAY)               goto LBL_ERR;
+      }
+      /* A = A/2, B = B/2 */
+      if ((err = mp_div_2(&A, &A)) != MP_OKAY)                    goto LBL_ERR;
+      if ((err = mp_div_2(&B, &B)) != MP_OKAY)                    goto LBL_ERR;
+   }
+
+   /* 5.  while v is even do */
+   while (MP_IS_EVEN(&v)) {
+      /* 5.1 v = v/2 */
+      if ((err = mp_div_2(&v, &v)) != MP_OKAY)                    goto LBL_ERR;
+
+      /* 5.2 if C or D is odd then */
+      if (MP_IS_ODD(&C) || MP_IS_ODD(&D)) {
+         /* C = (C+y)/2, D = (D-x)/2 */
+         if ((err = mp_add(&C, &y, &C)) != MP_OKAY)               goto LBL_ERR;
+         if ((err = mp_sub(&D, &x, &D)) != MP_OKAY)               goto LBL_ERR;
+      }
+      /* C = C/2, D = D/2 */
+      if ((err = mp_div_2(&C, &C)) != MP_OKAY)                    goto LBL_ERR;
+      if ((err = mp_div_2(&D, &D)) != MP_OKAY)                    goto LBL_ERR;
+   }
+
+   /* 6.  if u >= v then */
+   if (mp_cmp(&u, &v) != MP_LT) {
+      /* u = u - v, A = A - C, B = B - D */
+      if ((err = mp_sub(&u, &v, &u)) != MP_OKAY)                  goto LBL_ERR;
+
+      if ((err = mp_sub(&A, &C, &A)) != MP_OKAY)                  goto LBL_ERR;
+
+      if ((err = mp_sub(&B, &D, &B)) != MP_OKAY)                  goto LBL_ERR;
+   } else {
+      /* v - v - u, C = C - A, D = D - B */
+      if ((err = mp_sub(&v, &u, &v)) != MP_OKAY)                  goto LBL_ERR;
+
+      if ((err = mp_sub(&C, &A, &C)) != MP_OKAY)                  goto LBL_ERR;
+
+      if ((err = mp_sub(&D, &B, &D)) != MP_OKAY)                  goto LBL_ERR;
+   }
+
+   /* if not zero goto step 4 */
+   if (!MP_IS_ZERO(&u)) {
+      goto top;
+   }
+
+   /* now a = C, b = D, gcd == g*v */
+
+   /* if v != 1 then there is no inverse */
+   if (mp_cmp_d(&v, 1uL) != MP_EQ) {
+      err = MP_VAL;
+      goto LBL_ERR;
+   }
+
+   /* if its too low */
+   while (mp_cmp_d(&C, 0uL) == MP_LT) {
+      if ((err = mp_add(&C, b, &C)) != MP_OKAY)                   goto LBL_ERR;
+   }
+
+   /* too big */
+   while (mp_cmp_mag(&C, b) != MP_LT) {
+      if ((err = mp_sub(&C, b, &C)) != MP_OKAY)                   goto LBL_ERR;
+   }
+
+   /* C is now the inverse */
+   mp_exch(&C, c);
+   err = MP_OKAY;
+LBL_ERR:
+   mp_clear_multi(&x, &y, &u, &v, &A, &B, &C, &D, NULL);
+   return err;
+}
+#endif